ECE 530 – Analysis Techniques for Large-Scale Electrical Systems Prof. Hao Zhu Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign 10/12/ Lecture 12: Advanced Power Flow Topics

Announcements Midterm on Wed Oct 21, same room; 1 letter-size formula sheet + regular calculator Review session on Fri Oct 16, same room Homework 4 posted, due Oct 19 2

Transformer Modeling If transformers have a turns ratio (tap setting) that matches the ratio of the per unit voltages, then they are modeled in a manner similar to transmission lines. However it is common for transformers to have a variable turns ratio; this is known as an “off-nominal” tap Phase shifting transformers are also used sometimes, in which there is an angle shift across the transformer 3

The one–line diagram of a branch with a variable tap transformer The network representation of a branch with off– nominal turns ratio transformer is Transformer Representation k m the tap is on the side of bus k k t :1 4

Transformer Representation In the p.u. system, the transformer ratio is 1:1 if and only if this ratio equals the system nominal voltage ratio For off-nominal conditions, the turns ratio is obtained as a ratio of two p.u. quantities 5

An ideal transformer with t as the tap setting value Ideal Transformer 6

Transformer Nodal Equations From the network representation Also, 7

Transformer Nodal Equations We may rewrite these two equations as This approach is presented in F.L. Alvarado, “Formation of Y-Node using the Primitive Y-Node Concept,” IEEE Trans. Power App. and Syst., December

 –equivalent Circuit for Transformer Branch k m 9

Five Bus PowerWorld Example 10 PowerWorld Case: B5_Voltage With z=j0.1 between buses 4 and 5, the y node primitive with t=1.0 is If t=1.1(tap on bus 5) then it is

Variable Tap Transformer A transformer with a variable tap, i.e., the variable t is not constant, may be used to control the voltage at either the bus on the side of the tap or at the bus on the side away from the tap This is an example of single criterion control since we adjust a single control variable– the transformer tap t – to achieve a specified criterion: the maintenance of a constant voltage at a designated bus 11

Variable Tap Voltage Control A typical power transformer may be equipped with both fixed taps, on which the turns ratio is varied manually at no load, and automatic on-load tap changing (OLTC) or variable tap ratio transformers For example, the high-voltage winding might be equipped with a nominal voltage turns ratio plus four 2.5% fixed tap settings to yield  5% buck or boost voltage capability 12

Variable Tap Voltage Control In addition to this, there may be on the low-voltage winding, 32 incremental steps of 0.625% each, giving an automatic range of  10% It follows from the  – equivalent model for the transformer that the transfer admittance between the buses of the transformer branch and the contribution to the self admittance at the bus away from the tap explicitly depend on t However, the tap changes in discrete steps; there is also a built-in time delay in how fast they respond Voltage regulators are devices with a unity nominal ratio, and then a similar tap range 13

Ameren Champaign Test Facility Voltage Regulators 14 These are connected on the low side of a 69/12.4 kV transformer; each phase can be regulated separately

Variable Tap Voltage Control OLTCs (or voltage regulators) can be directly included in the power flow equations by modifying the Y bus entries; that is by scaling the terms by 1, 1/t or 1/t^2 as appropriate If t is fixed then there is no change in the number of equations If t is variable, such as to enforce a voltage equality, then it can be included either by adding an additional equation and variable (t) directly, or by doing an “outer loop” calculation in which t is varied outside of the NR solution 15

Outer Loop Tap Control The challenge with implementing tap control in the power flow is it is quite common for at least some of the taps to reach their limits – Keeping in mind a large case may have thousands of LTCs! If this control was directly included in the power flow equations then every time a limit was encountered the Jacobian would change – Also taps are discrete variables, so voltages must be a range Doing an outer loop control can more directly include the limit impacts; usually sensitivity values are used in the calculation 16

Phase-Shifting Transformers Phase-shifters are transformers in which the phase angle across the transformer can be varied in order to control real power flow – Sometimes they are called phase angle regulars (PAR) – Quadrature booster (British usage) They are constructed by include a delta- connected winding that introduces a 90º phase shift that is added to the output voltage Image: 17

Phase-Shifter Model We develop the mathematical model of a phase-shifting transformer as a first step toward our study of its simulation Let buses k and m be the terminals of the phase–shifting transformer The latter differs from an off–nominal turns ratio transformer in that its tap ratio is a complex quantity, i.e., a complex number 18

Phase-Shifter Model For a phase shifter located on the branch (k, m), the admittance matrix representation is obtained analogously to that for the LTC Note, if there is a phase shift then the Ybus is no longer symmetric!! In a large case there are almost always some phase shifters 19

Phase-Shifter Representation one-line diagram of a branch with a variable phase shifting transformer network representation of a branch with a variable phase shifting transformer k : 1 m k m tap is on the side of bus k 20

Integrated Phase-Shifter Control Phase shifters are usually used to control the real power flow on a device Similar to OLTCs, phase-shifter control can either be directly integrated into the power flow equations (adding an equation for the real power flow equality constraint, and a variable for the phase shifter value), or they can be handled in with an outer loop approach As was the case with OLTCs, limit enforcement often makes the outer loop approach preferred 21

Two Bus Phase Shifter Example 22 PowerWorld Case: B2PhaseShifter Top line has x=0.2 pu, while the phase shifter has x=0.25 pu.

Switched Shunts and SVCs Switched capacitors and sometimes reactors are widely used at both the transmission and distribution levels to supply or (for reactors) absorb discrete amounts of reactive power Static Var compensators (SVCs) are also used to supply continuously varying amounts of reactive power In the power flow SVCs are sometimes represented as PV buses with zero real power 23

Switched Shunt Control The status of switched shunts can be handled in an outer loop algorithm, similar to what is done for OLTCs and phase shifters – Because they are discrete they need to regulate a value to a voltage range Switched shunts often have multiple levels that need to be simulated Switched shunt control can interact with the OLTC and SVC control The power flow modeling needs to take into account the control time delays associated with the various devices 24